D-psicose, an epimer of D-fructose, is very similar to D-fructose in terms of the intensity and the variety of sweetness. However, unlike D-fructose, D-psicose is hardly metabolized upon being absorbed in the body, and has a low caloric contribution. PTL 1 describes advantageously using D-psicose as a low calorie sweetener for production of low calorie food and drinks. That is, D-psicose has potential use as an active ingredient of diet food. Sugar alcohols, widely used as non-sugar sweeteners, cause side effects such as diarrhea when ingested above the specified amounts. D-psicose has fewer such side effects. Further, because D-psicose is hardly metabolized in human body, the caloric value is nearly zero, and the sugar suppresses the lipid synthetase activity and lowers abdominal fat. This makes D-psicose also useful as a sweetener beneficial to body weight reduction (see, for example, NPL 1 and NPL 2). PTL 2 describes that potentially advantageous use of D-psicose can be advantageously used for products such as food with health claims, food and drinks for diabetes patients, and slimming food and drinks because D-psicose has the blood sugar level suppressing effect. From these viewpoints, D-psicose has attracted a great deal of interest as a food with health claims, and a diet sweetener, and there is a growing need for the development of an efficient and safe method of producing D-psicose in food industry.
NPL 3 discloses a D-ketose 3-epimerase derived from Pseudomonas cichorii ST-24, and using this enzyme for production of D-psicose from D-fructose. However, as the alternative name D-tagatose 3-epimerase suggests, the enzyme has the highest specificity for D-tagatose, and its activity on D-fructose is known to be relatively weak. Further, being a plant pathogen, the Pseudomonas cichorii has a very poor capability for producing D-ketose 3-epimerase, and is not suited for industrial use. There is accordingly a need for a microorganism different from Pseudomonas, and that has a high ketose 3-epimerase production capability for safe food production. A novel ketose 3-epimerase having high specificity for D-fructose, and suited for D-psicose production is also needed.
PTL 3 attempts to provide a novel ketose 3-epimerase, a method of production thereof, a DNA encoding the enzyme, a recombinant DNA and a transformant containing the DNA, and a method for producing ketose with the enzyme. This publication actually provides a ketose 3-epimerase that can be obtained from microorganisms of the genus Rhizobium, a method of production thereof, a DNA encoding the enzyme, a recombinant DNA and a transformant containing the DNA, and a method that uses the enzyme to epimerize a D- or L-ketose at position 3 and produce a corresponding D- or L-ketose. PTL 4 develops a method for producing D-psicose with a D-psicose epimerase (hereinafter, “psicose 3-epimerase”) derived from Agrobacterium tumefaciens. 
As described above, the foregoing problems are addressed by the studies of methods that enzymatically produce D-psicose by using D-fructose as a substrate. However, the methods developed to date are problematic in terms of the safety of D-psicose production for food.